There's no facility to be able to directly send an SMS via GV with sipsorcery. I suspect there are other services or sites that do provide this and SMS providers like clickatell.com definitely do although that's a paid service.

You can put the sys.GoogleVoiceSMS call into your webcallback dialplan and initiate it from a URL request but that seems like a very roundabout way to do it. For more information on setting up a webcallback dialplan see http://www.sipsorcery.com/mainsite/Help/WebCallback.